Post History

Make it possible to toggle between markdown source and rendered output.  This is helpful for the history view but essential for reviewing suggested edits:

- If we only had rendered output and not markdown, then it would be much harder to catch sneaky spam injections like making a `.` a link.

- If we only have markdown (like now), then reviewing formatting-heavy posts, including anything with Mathjax, is daunting.  I don't know how folks on our Mathematics community even manage that.  We shouldn't make them work that hard.

For a post history this should be a toggle; we don't want to double the size of that page by always showing both.  While there's more room on a suggested-edit page, I think it should be a toggle there too, at least for now, instead of trying to show both source and rendered output.  You want both to be available when you're *creating* content, which is why we have the preview, but for *reviewing*, let's not add the extra UI and code complexity until we determine that it's needed.  (And also, let's not forget about phones.)